GPS/INS integrated systems, which are integration of a GPS receiver and inertial sensors and calculate positions, velocities of a moving body, can calculate more robust and highly accurate positions and velocities by integrating outputs from the GPS receiver and outputs from the inertial sensors.
The outputs from the GPS receiver includes pseudorange measurements and Doppler frequency measurements generated by the GPS receiver (hereinafter, referred to as “GPS measurements”), positions, and velocities (hereinafter, referred to as “GPS positions,” “GPS velocities,” respectively) calculated based on ephemeris data or the like.
In order to obtain reliable outputs from the GPS receiver, it is necessary to detect the existence of normality or abnormality of the GPS measurements, and eliminate the abnormal value. Typically, although the GPS receiver eliminates the abnormal GPS measurements based on the normality/abnormality determination of the received GPS signals, they are unable to be eliminated completely in actual cases.
Therefore, it is necessary to determine the abnormality in the outputs from the GPS receiver as well as for the outside of the GPS receiver. The conventional methods of the abnormality determination includes a method based on a difference between a pseudorange calculated from a PN code and it estimated value as disclosed in the Patent Document 1, a method based on a difference between a GPS position and a position obtained from an external device as described in the Patent Document 2, and a method based on a difference between a pseudorange calculated from a PN code and a distance calculated from a carrier phase as described in the Patent Document 3, etc.
Because the method described in the Patent Document 2 requires the external support devices other than GPS, it will be expensive and their applications may be restricted. The method as described in the Patent Document 3 is effective in detection where, for example, the detection is more strongly influenced with one of the signs of multipath errors. However, the methods of detecting the abnormality by a difference will loose their advantages when influenced by reversed polarity due to, such as ionosphere errors or when there are errors which receive the same kind of influences.
The method of the Patent Document 1 is a well-used technique, as seen in the Receiver Autonomous Integrity Monitoring (RAIM) technique.
[Patent Document 1] Masato Kawai, Shigeki Hara, and Osamu Arai, “A New Step Detector Algorithm For GPS/SBAS Receiver” ION GPS 2001, 14th International Technical Meeting of the Satellite Division of the Institute of Navigation, Sep. 11-14, 2001
[Patent Document 1] JP2004-301725(A)
[Patent Document 2] U.S. Pat. No. 5,917,445